ou already know it's hard to balance your checkbook while simultaneously reflecting on your past. Now, investigators at the Stanford University School of Medicine -- having done the equivalent of wire-tapping a hard-to-reach region of the brain -- can tell us how this impasse arises. The researchers showed that groups of nerve cells in a structure called the posterior medial cortex, or PMC, are strongly activated during a recall task such as trying to remember whether you had coffee yesterday, but just as strongly suppressed when you're engaged in solving a math problem.